Sunday, January 31, 2021

Lens Transmissions of specialized Lenses for reflected UV Photography

So today my first post in 2021 about some UV-VIS-NIR measurements of some highly specialized lenses, which were specifically developed for reflected ultraviolet (UV) photography.

Usually a photographic lens should reproduce visible images, which is the wavelength band of about 400-700nm. This is why lens makers apply a special multicoating (aside from achieving higher light transmission), which usually blocks light outside this visible waveband. For biological reflected UV photography however, the UV-A band (300 - 400nm) is the interesting one. So using such a modern UV lens and that (de facto "standard" UV transmitting) Baader U-filter  will work very well together, if both lens + filter have a high enough transmission. Culprit is, that coating for such a wide range is tricky, so quite a few of those lenses are still uncoated.

Here now I have used my UV-VIS-NIR spectrometer system consisting of a stabilized Xenon light source connected with a quartz fiber to a quartz-fluorite condensor which generates a fine parallel output ray shining through the lens to be tested and a receiving Spectralon (R) coated integrating sphere which captures all the light of that diverging beam exiting the measured lens and transmits this via a quartz fiber to the digital spectrometer.

The following lenses have been measured (links lead to my macrolenses site with lens data):

- CoastalOpt (Jenoptik) f4 / 60mm UV-VIS-NIR Macro Apo (Nikon-F mount)

- UV-Nikkor f4.5 / 105mm (Nikon-F mount)

 (identical to the RAYFACT f4.5 105mm sold by Nikon TOCHIGI)

- Lavision f2.8 / 85mm UV (Nikon-F mount)

- Hamamatsu A4869 f3.5 / 50mm (c-mount)

 [click on image to see a larger one]

All these lenses show a rather high UV transmisison, the CoastalOpt 60mm especially, as it seems to have a modern BBAR coating, but its downside is that its transmission only starts from about 320nm onwards, suitable for UV-A photography for biologic studies, but less suitable for technical studies like combustion analysis etc. which demands a suitable transmission in the UV-B (also sometimes UV-C) range which the other three lenses are capable of, reaching down to about 200nm.

The Hamamatsu 50mm is a c-mount lens and covers 1" sensors, the Lavision 85mm covers APS-C format. UV-Nikkor 105mm and CoastOpt 60mm are full format lenses.

Just a remark, as a high UV transmission is a must for a suitable UV lens, but also good sharpness, high contrast, flare resistance, no hot-spots, etc. are requirements such a UV lens has to meet, and I have not jet mentioned an important point - focus shift. Most of these modern quartz-fluorite lenses have no or very little focus shift, but that depends on the wavelength range they are corrected for. 

There ia a part II of this covering 85mm lenses for UV HERE

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site